The ratio of the amounts of U 238 and Pb 206 in a rock sample enables the age of the rock to be estimated using the technique of radiometric dating. It moves back in the periodic table until the isotope falls in the band of stability at Pb 206.
A base turns red litmus blue. Litmus paper is paper that has been treated with a specific indicator, a mixture of 10 to 15 natural dyes that turns red in response when the pH is acidic, and blue when it is basic.